1. Field of the Invention
The present invention relates to a surface mounting type semiconductor chip package and, more particularly, to a solder structure for a BGA (Ball Grid Array) package or LGA (Land Grid Array) package and a method of fabricating the same.
2. Discussion of the Related Art
FIG. 1 illustrates a conventional BGA package showing an array of solder balls 8 provided, in place of leads, on the bottom surface of an interconnection substrate of a package body 2. The conventional BGA package body occupies a smaller area than a QFP (Quad Flat Package) type, and is advantageous because the conventional BGA package shows no distortion of the leads, in contrast to the QFP type.
The steps of fabricating the conventional BGA package are outlined as follows.
After fabricating integrated circuits on a wafer, semiconductor chips 1 on the wafer are severed by sawing the wafer. Then a process of forming an interconnection substrate of a package body is performed and thus, the interconnection substrate having an interconnection therein is provided. On the upper surface of the interconnection substrate, a coat of adhesive 9 is applied to bond the severed semiconductor chip 1 thereon. After bonding the semiconductor chip 1 to the interconnection substrate, a wire bonding process is performed in which bonding pads 10 formed on the semiconductor chip 1 and respective interconnections 11 are electrically connected with fine metal lines 12.
After the wire bonding, the semiconductor chip 1 is molded using an EMC (Epoxy Molding Compound) 13. After the molding process is completed a flux coating step is performed using screen printing to transcribe a solder paste of a predetermined pattern so as to coat a flux 14 on the bottom of the interconnection substrate of the package body 2. After the flux coating, the solder balls 8 are attached to the coated flux 14. The solder balls 8 are then reflowed to fix the solder balls 8 to the package body 2 firmly. Thereafter, through cleaning and marking, a completed BGA package is produced. However, if the thermal expansion coefficients of the package body 2 of the interconnection substrate and a mounting board (not shown) are different from each other, it will have a negative effect on the package body 2.
FIG. 2 illustrates a BGA package disclosed in a Japanese Laid Open Patent No. H8-46084 and FIG. 3 shows an enlarged view of a portion A of FIG. 2. This BGA package is described, for preventing negative effects on the package body 2 due to the difference between the thermal coefficients of the package body 2 and the mounting board.
Referring to FIGS. 2 and 3, the BGA package includes at large a package body 2 having interconnections formed therein, interconnection pattern films 16 bonded to the bottom of the package body 2 using an elastic layer 15, and solder balls 8 formed on the films 16 as external connection terminals. Each of the interconnection pattern films 16 in turn includes an electrically insulating base film 17 and an interconnection pattern 18. One end of the interconnection pattern 18 being in direct contact with the base film 17 is connected to the respective solder ball 8, and the other end of the interconnection pattern 18 is electrically connected to an internal interconnection 11 formed within the package body 2.
In operation, the elastic layer 15 of the BGA package having the aforementioned system compensates for the difference between the thermal expansion coefficients of the package body 2 and the mounting board, and thus protects the solder joint. The fabrication process for the BGA package disclosed in the Japanese Laid Open Patent No. H8-46084 is identical to the above-described fabrication process of the conventional package, except for the step of attaching the interconnection pattern film 16 and the elastic layer 15.
However, the step of attaching the solder balls 8 in the conventional BGA packaging process causes a problem of reduced productivity. The conventional BGA packaging process has problems in that to perform the solder ball attaching step, a solder ball attaching equipment is needed, which increases the equipment cost and lowers the investment efficiency. Further, if one of the solder balls 8 soldered on the package body 2 were to fall off from the attached surface, rebonding of the fallen off solder ball is extremely difficult. This will inevitably increase defects in the package.
Moreover, there is a limitation on increasing the stand off height ("h" in FIG. 2) of the solder ball 8 in the conventional method of attaching the solder ball 8. This limitation places restrictions on improving the solder joint reliability and the subsequent package mounting reliability of the BGA package. If the thermal expansion coefficients of the package body 2 and the mounting substrate were not identical, the shearing force is exerted on a jointing part between the solder ball 8 and the mounting substrate due to the difference in the thermal expansion coefficient. The shearing force will eventually break the solder joint during the operation of the BGA package mounted on a mounting board because the value of the stand off height for the solder ball 8 is low. Consequently, the conventional method of joining solder balls has a problem of considerably reducing the life span of the semiconductor package.